This application claims priority to International Patent Application Serial No. PCT/US2013/030871, filed Mar. 13, 2013, which is incorporated by reference herein in its entirety.
Pressure pumping is an integral part of well services such as stimulation and cementing that deliver required fluids using pumping equipment of a well servicing system at high pressures and flow rates. It is therefore of the utmost importance that the integrity of the well servicing system is tested and maintained.
Hydrostatic pressure testing is one type of pressure testing method, and is a way in which pressure containing components in a well servicing system (land-based or offshore) can be tested for their integrity. For offshore pressure pumping, a significant amount of the equipment's operational time is spent performing hydrostatic pressure tests. During a hydrostatic pressure test, a well servicing system (defined from here forth as components forming a fluid delivery or containment system, such as pumping equipment, pump fluid end, treating lines, rig treating lines, well head, casing, tubular, open hole formation) is subjected to a specific target internal pressure and the pressure is held in this closed and contained system for a predetermined period of time to reassure its strength or to detect leaks, if any. The system is contained by means such as using closed valves in the treating lines or the casing and down-hole formation. A hydrostatic pressure test is typically carried out using pressure pumps, e.g., positive displacement pumps, in fluid communication with the system to pressurize this system to a test pressure. During this test, a testing medium is pumped into the system by the pressure pumps until the system is fully primed. In order to build up pressure in this system volume, the pump further displaces and compresses the medium. Once the target pressure is reached, the pressure is held for a specified time.
In order to verify the integrity of the system, pressure tests up to the rated working pressure of certain components is highly desirable, in part, to ensure that no incident of over-pressuring occurs, thereby compromising safety and leading to costly downtime to conduct repairs or replace components. This is even more so, considering the high frequency of hydrostatic tests and the potential high pressures involved during such testing.
Pumping equipment may consist of a primary mover that is either a Reciprocating Internal Combustion (RIC) engine, an electric motor, or the like, which is coupled to a transmission or gear box for speed and torque control, and which further drives a high pressure pump. The primary limitations of the RIC engine compared to the electric motor in terms of pressure testing, are (1) the RIC engine cannot operate at low speeds and hence have low resolution in terms of pressure increase at the driven pump, and (2) the RIC engine is typically coupled to a transmission that has a minimum allowable input speed, hence contributing to low resolution if this speed is too high. Some variants of pumping equipment with electric motors as primary movers also employ these transmissions, hence creating the same issue as described in (2).
The most widely used method for pressure testing systems that have a transmission is a trial-and-error approach, where the primary mover is set at idle speed (for an electric motor, this speed will correspond to the minimum allowable input speed to the transmission) and subsequently ‘bumping’ by moving from in-gear to neutral repetitively to increase pressure. If the pressure in the system is still inadequate, the primary mover's torque is typically increased by increasing its speed. Preventing any over-pressure incident in this widely used method depends on how quickly the pump can be stopped by shifting the transmission to neutral when the desired test pressure is reached. This depends on both the ability of the equipment operator and the characteristics of the equipment. In terms of the equipment, there are three primary factors that affect the controllability of the method: (1) the behavior and torque multiplication of the transmission in torque converter (fluid coupling) mode is difficult to predict as it is a function of a number of parameters such as torque converter geometry and oil characteristics; (2) the ‘bumping’ method uses momentum to increase the pressure, and this is highly unpredictable and can cause significant damage to the transmission after repeated tests; and (3) the nature of the reciprocating pump means that the load is varying as a function of the crank angle, implying that even if the exact same steps are taken for every test, the resultant pressure increase will be different if the crank angle is not the same every single time. The current industry trend of requiring higher powered pumping equipment especially for deepwater rigs, further compounds these issues.
A few scenarios that are inadequately addressed by current methods are (1) low pressure tests (e.g., 500 pounds per square inch (psi)) where the larger the equipment, the lower the resolution in terms of pressure increase which may lead to single bumps of even higher than 500 psi, (2) small volume tests (e.g., local treating lines of the pumping equipment), and (3) reaching close to the target pressure but not yet within the allowed +/− acceptance range while not being able to confidently take the next step for fear of over-pressuring. Currently, Over-Pressure Shut-Down (OPSD) systems are used to address this, however this method is more of a ‘reactive’ approach and requires very quick response times that may not be practical or achievable.